Primidone
MYSOLINE
Pronunciation: PRI-mi-doen
“Prima donna’s Missile line”
“Shredder” inducer
Metabolized to phenobarbital, similar theme
Pan-inducer, affecting substrates shown on the paper
What it isDeoxybarbiturate antiepileptic that is active on its own; only 15–25% is oxidized to phenobarbital, most of a dose leaves unchanged, and the largest metabolite, PEMA, is inert in tremor.
Why this oneWith propranolol it is one of only two first-line treatments for essential tremor, and the tremor effect is primidone’s own: one dose quiets tremor for up to a day while phenobarbital and PEMA are still barely measurable. Not a controlled substance.
What limits itA severe acute reaction on the first dose is common, so start at a fraction of the target. It is an enzyme inducer through its phenobarbital metabolite.
FDA-approved for
- seizures (~1954)
Off-label
- essential tremor
FDA label
Read the full label on DailyMed ↗
| FDA dosing | Population | Start | Target | Max |
|---|---|---|---|---|
| Epilepsy (grand mal, psychomotor, focal), no prior treatment | Adult & child ≥8 yr | 100-125 mg QHS | 250 mg TID | 500 mg QID (2000 mg/day) |
| Epilepsy | Pediatric <8 yr | 50 mg QHS | 125-250 mg TID (or 10-25 mg/kg/day divided) | 250 mg TID |
Titration Increase approximately every 3 days per initiation schedule (Days 1-3, 4-6, 7-9, then maintenance)
Renal Undefined
Hepatic Undefined
Target serum primidone level 5-12 mcg/mL
Instructions
- When adding to another anticonvulsant, start 100-125 mg QHS and titrate up as the other drug is tapered; complete transition over ≥2 weeks
Available strengths
- tab 50, 125, 250 mg
Contraindications
- Porphyria
- Hypersensitivity to phenobarbital
Cautions
- Early ataxia and vertigo (usually transient)
- Sedation/drowsiness
- Rare blood dyscrasias (granulocytopenia, agranulocytosis, red-cell aplasia)
- Megaloblastic anemia (responds to folic acid)
Adverse reactions
- Ataxia, vertigo
- Nausea, anorexia, vomiting
- Fatigue, drowsiness, hyperirritability
- Diplopia, nystagmus
- Rare agranulocytosis, megaloblastic anemia, morbilliform rash
Pregnancy Undefined
Lactation Undefined
Cost
Generic entry2001
NO BOXED WARNINGS
Principal riskSedation; converts to phenobarbital
Classes and tags
Clinical profile
Legacy pregnancy categoryD
Defining liabilitySedation, dependence and phenobarbital-mediated respiratory/CNS depression in overdose.
Active metabolites
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